There have been various proposals for a cylinder unit used in a hydraulic shock absorber, a gas spring, and the like until now. A cylinder unit is interposed, for example, between a chassis and an axle of an automobile to absorb a vibration inputting to a wheel on an uneven road surface.
JP05-321969A discloses a conventional cylinder unit. This cylinder unit includes a cylinder, an annular rod guide provided in one end side of the cylinder, a piston having an outer circumference slidably adjoining an inner circumference of the cylinder to divide an inner space of the cylinder into a pair of chambers, and a piston rod movably inserted into the cylinder by interposing a piston so as to pass through a rod guide.
The cylinder unit has a rebound member provided between the rod guide and the piston to absorb impact generated when the cylinder unit is fully extended. The rebound member includes a coil spring arranged around an outer circumference of the piston rod and a pair of upper and lower holder members fitted to fixing portions formed in both ends of the coil spring.
The upper holder member is slidably provided in an outer circumference of the piston rod. The lower holder member is forcibly inserted into the outer circumference of the piston rod and is fixed vertically over the piston.
In a case where the cylinder unit is extended, the piston rod exits from the cylinder, and the piston approaches the rod guide. When the cylinder unit is fully extended, the coil spring is compressed to absorb impact caused by the fully extension.
As illustrated in FIG. 4, the coil spring 600 is a wire rod having a circular cross section and including coil portions 600a, 600b, 600c, and 600d forming to be continuously connected in an axial direction. When the cylinder unit is fully extended, the vertically neighboring coil portions abut on each other to make a fully compressed state. In this case, the neighboring coil portions may slip from each other, and a part of the coil springs 600b may be moved by a distance f2 in a radial direction so as to generate an abnormal noise.
In this regard, the applicant proposed a technique of preventing a slide of the coil portion by making the cross section of the coil spring, for example, in a rectangular shape as disclosed in JP 2006-132620A.